The present invention is directed to car radio equipment which is known in practice and is based on “21C technology” in which the audio playback device and its tuner in particular are equipped with a digital receiver unit in particular (known as a “digiceiver”). In other words, this means that, for example, the high-frequency signal of the IF (intermediate frequency) stage at 10.7 megahertz is converted into bits and bytes as early as in the reception part of the tuner, and following this, it is processed further consistently on a digital level up to the output stages.
In such car radio equipment, for example, two or three freely programmable audio filters are integrated into the signal path. These digital parametric equalizers (DPEs) are available to the user to compensate for inadequacies in the acoustics in the interior of the vehicle. The user is able to adjust certain parameters of each filter, e.g., with respect to damping or quality, i.e., filter width, mid-frequency and/or gain to compensate for overshooting and holes, as they are called, in the acoustic frequency response of the interior of the vehicle.
In conjunction with the acoustic frequency response or, in more general terms, the loudness perception of the human ear, it should be recalled that this loudness perception varies over the audible frequency range. This means, in other words, that the sensitivity of the human ear is not constant over all frequencies but instead decreases toward high frequencies and declines to an even greater extent toward low frequencies. This effect is even more pronounced at lower sound pressure levels and, as shown in FIG. 1, it is described in the literature by curves of equal loudness (known as isophones) plotted as sound pressure level L (in dB=decibel) as a function of frequency f (in Hz=hertz) (see also ISO (International Standardization Organization) 226).
At an audio frequency of one hundred hertz, for example, a much higher sound pressure level is necessary to achieve the same loudness impression as at a frequency of one kilohertz. This relationship is illustrated by the plot of the aforementioned lines of equal loudness in a sound pressure/frequency diagram (see FIG. 1, where the speech range has been labeled as 80, the music range as 82 and the audible threshold as 84).
To compensate for this effect, audio signal playback systems often have a device for selective boosting of the bass level at a low playback loudness. In many systems, the highs are also boosted slightly. This device, which is known as “acoustically correct loudness correction” or “loudness control” is intended to keep the loudness of the audio signal perceived by the listener constant over the entire audible frequency spectrum, regardless of playback loudness, i.e., to ensure a spectrally balanced sound.
Therefore, many systems have implemented a loudness function to ensure that the sound impression, e.g., as the balance between low frequency components and medium frequency components, remains uniform at all loudness levels in an audio playback system.
This loudness function also alters the tone adjustment in conjunction with loudness, so that all frequency components are perceived as equally loud. In particular, in a simple embodiment the low frequency components below a certain use frequency are boosted with the help of a sound control unit at a reduced loudness; this then constitutes the main component of an acoustically correct correction of frequency response. This use frequency is either set at the factory in each audio playback system or it may be manually adjusted by the user within certain limits.
However, in the case of known loudness functions, the choice of an optimum use frequency for boosting the bass level poses a problem. Since the sensitivity of the human ear declines greatly at low frequencies, it is usually necessary to boost the bass level to extremely low frequencies in order to achieve an acoustically correct correction.
However, this is not appropriate in all audio playback systems, because the performance of certain amplifiers and/or loudspeakers potentially does not allow playback of extremely low frequencies. Greater boosting at very low frequencies would thus have no acoustic effect and might under some circumstances even result in overloading the amplifier stages and/or loudspeakers.
For such systems, it is advisable to choose a higher use frequency and/or to limit boosting toward low frequencies in order to achieve at least an acoustically correct correction for the upper bass frequency range without overloading the playback system. If the performance of the amplifier stages and/or loudspeakers is not known by the control unit responsible for the loudness, which is typically the case in a system having a “car radio booster loudspeaker,” the adjustment of the loudness characteristic at the factory may not always be optimal.
Manual adjustment of the loudness characteristic by the user, however, requires a certain amount of acoustic and technical knowhow. In particular, such a manual adjustment proves problematical in practice inasmuch as the user must be very familiar with the acoustics of his vehicular device to make an optimum adjustment of the equalizer, and it is very difficult to determine the acoustic frequency response without the help of measurement technology, i.e., merely by listening.
The operating instructions of known car radio equipment may provide only very limited assistance in making the best possible adjustment of the equalizers, because these operating instructions are by no means able to take into account all makes and models of vehicles, let alone the multitude of individual equipment options and amplifier and loudspeaker configurations.
Furthermore, car radio equipment having an audio module which is integrated into the signal path and has a graphic equalizer implemented on it with the help of a digital signal processor, are also known. The seven or nine bands of such a graphic equalizer have a fixed mid-frequency and only the gain is adjustable. The separate audio module with such car radio equipment permits automatic calibration of the graphic equalizer.
To do so, the acoustics in the interior of the vehicle is measured with the help of a microphone connected to the audio module via an AD (analog/digital) converter. With the help of special software, the graphic equalizer is then adjusted to compensate as much as possible for the acoustic inadequacies.
Use of a graphic equalizer to optimize the acoustically correct boosting of the bass level has proven to be problematical in practice with respect to the acoustics of the interior of a vehicle. As mentioned above, the mid-frequencies of the equalizer bands of a graphic equalizer are fixed, usually being a minimum interval of one octave in the case of nine bands. Thus, narrow resonance overshooting in the acoustic frequency response of the interior of the vehicle between the equalizer bands will not be optimally compensated. Furthermore, the additional audio module having the digital signal processor for implementing the graphic equalizer and for calibration of this equalizer are relatively cost intensive.